What language are the shaders in Blender written in?
Most of them are written in OSL, OpenShadingLanguage, a language specifically designed for shaders. It's similar to C and can be compiled, so that it's fast.

You can find it in the Blender sources at

intern\cycles\kernel\shaders\node_principled_bsdf.osl


OSL is comparable to RSL, Renderman Shading Language and GLSL, OpenGL Shader Language.